Electrochemical sensors have long been used to detect or measure the presence of substances in fluid samples. Electrochemical sensors include a reagent mixture containing at least an electron transfer agent (also referred to as an “electron mediator”) and an analyte specific bio-catalytic protein (e.g. a particular enzyme), and one or more electrodes. Such sensors rely on electron transfer between the electron mediator and the electrode surfaces and function by measuring electrochemical redox reactions. When used in an electrochemical biosensor system or device, the electron transfer reactions are monitored via an electrical signal that correlates to the concentration of the analyte being measured in the fluid sample.
The use of such electrochemical sensors to detect analytes in bodily fluids, such as blood or blood derived products, tears, urine, and saliva, has become important, and in some cases, vital to maintain the health of certain individuals. In the health care field, people such as diabetics, for example, must monitor a particular constituent within their bodily fluids. A number of systems are capable of testing a body fluid, such as, blood, urine, or saliva, to conveniently monitor the level of a particular fluid constituent, such as, cholesterol, proteins, and glucose. Patients suffering from diabetes, a disorder of the pancreas where insufficient insulin production prevents the proper digestion of sugar, have a need to carefully monitor their blood glucose levels on a daily basis. Routine testing and controlling blood glucose for people with diabetes can reduce their risk of serious damage to the eyes, nerves, and kidneys.
Electrochemical biosensors may be adversely affected by the presence of interfering substances in the test sample that may undesirably affect the measurement and lead to inaccuracies in the detected signal. Another source of inaccuracy is from insufficient sample being applied to the test strip. These sources of inaccuracy may cause an inaccurate glucose reading, leaving the patient unaware of a potentially dangerous blood sugar level.
It is known, for example, in U.S. Pat. No. 5,582,697, to have an additional fill detection electrode located downstream from the working electrode to determine that a sufficient volume of fluid has been applied to the test strip. An additional electrode increases the size of the electrode cell which increases the size of the sample required to fill the cell.